Tankers and other cargo ships are furnished with ballast tanks for weighting and/or balancing the ship. The ballast tanks are filled with water when the ship is not loaded with oil or other cargo, and emptied when the ship is loaded. Thus is the buoyancy of the ship readjusted for optimal stability both when it carries cargo and when it does not.
Ballast water, essential as above for the safety of seagoing vessels, is normally the seawater taken in at the source ports where they are unloaded of their cargo. It is estimated that, worldwide, as much as well over 10,000,000,000 tons of seawater is being used annually for ballasting ships.
One of the most serious threats to the world's oceans today arises from the fact the ballast water being carried by ships unavoidably contains a variety of aquatic lifeforms which inhabited the area where the water was taken in. As the ships traverse the oceans, so do the organisms contained in their ballast water, eventually to be released into non-native environments upon discharge of the ballast water at the destination ports. Some invasive ones of these organisms may destroy and replace the native species in their new host environments.
In the light of such hazards to ecosystems caused by ballast water, a diplomatic conference at International Maritime Organization (IMO) adopted the International Convention for the Control and Management of Ships' Ballast Water and Sediments (hereinafter referred to as the Convention). The Convention requires all ships constructed in and after 2009 to implement a Ballast Water and Sediment Management Plan.
The ballast water performance standards of the Convention require that ships conducting ballast water management shall discharge:
TABLE 1Ballast WaterItemsQuality CriteriaSizeAquatic Organisms10unit/ml10-50 μmAquatic Organisms10unit/m350 μm or moreIndicatorEscherichia Coli250cfu/100 ml/MicrobesVibrio cholerae1cfu/100 ml/(O1 and O139)Genus Enterococcus100cfu/100 ml/
In short the ballast water performance standards require that the concentrations of viable organisms in ballast water should be reduced to approximately one hundredth of those in outer oceans. The advent of technologies that meet these stringent IMO standards have been awaited in the shipbuilding industry.
Japanese Unexamined Patent Publication No. 2003-200156 represents a conventional method of ballast water treatment. It teaches to pump the water into and through a slitted screen with a view to the mechanical destruction of the sealife contained. Although effective to a limited extent for incapacitating aquatic organisms in general, this known method does not work with microorganisms and so fails to come up to the IMO ballast water performance standards.
It might be contemplated to compensate for this weakness of the conventional method above by introduction of ozone into ballast water. Ozone introduction into the ballast water being pumped under high pressure has its own difficulties that must be overcome. The ballast pump in use must be capable of developing a pressure of 1.5 megapascals (MPa) or so in consideration of both the pressure loss ΔP of 0.5-1.0 MPa as the water travels through the slitted screen and the delivery pressure of 0.2-0.4 MPa needed for charging water into the ballast tanks.
Ozone generators or ozonizers available today, on the other hand, have a delivery pressure of 0.2 MPa at the maximum. Direct ozone introduction into the ballast water being pumped toward the ballast tanks is difficult because of the much higher delivery pressure of the ballast pump. An increase of the ozone introduction pressure to the required level would make the installation costs inordinately high.